A typical steam turbine is configured such that a rotor as a rotating shaft is rotatably supported by a casing, and that blades are provided on an outer circumferential portion of the rotor and vanes are provided in the casing so that the blades and the vanes are alternately arranged in a plurality of stages in a steam passage. Therefore, when steam flows in the steam passage, the flow of the steam is straightened by the vanes, so that the steam can drive and rotate the rotor through the blades.
In such a steam turbine, in a final stage of a cascade of a low-pressure turbine, the steam becomes wet steam mixed with water drops (drain). Therefore, a loss due to the wet steam occurs. Further, the water drops contained in the wet steam collide with the blades rotating at a high speed, so that erosion occurs in end portions of the blades.
To solve such a problem, a technology of heating vanes and an outer ring and an inner ring that support the end portions of the vanes is described in Patent Literatures below. In steam turbines described in Patent Literatures, steam is supplied to a hollow portion of the outer ring, supplied to a hollow portion of the inner ring through hollow portions of a plurality of vanes, returned to the hollow portion of the outer ring through the hollow portions of the plurality of vanes again, and then discharged. The vanes, the outer ring, and the inner ring are heated by the steam.